Technologies for removing NOx contained in the exhaust gas of an internal combustion engine by means of what is called an NOx storage reduction catalyst (which will be referred to as an “NOx catalyst” hereinafter) provided in an exhaust passage of the internal combustion engine and thereby purifying the exhaust gas have been known. However, when SOx contained in the exhaust gas is stored by the NOx catalyst to cause SOx-poisoning of the NOx catalyst, the NOx catalyst is deteriorated and the performance of removing NOx and purifying the exhaust gas is decreased.
As a solution for this, Japanese Patent Application Laid-Open No. 2001-20781 discloses a technology in which when an NOx catalyst is deteriorated for the reason of SOx-poisoning etc., a method of controlling the air-fuel ratio of the exhaust gas for removing NOx that is different from that used when the NOx catalyst is not poisoned is adopted. In this technology, when removing NOx, the air-fuel ratio of the exhaust gas is once adjusted to a predetermined rich air-fuel ratio and thereafter adjusted to an air-fuel ratio near the stoichiometric air-fuel ratio. In addition, the time over which the stoichiometric air-fuel ratio is maintained is varied depending on whether the NOx catalyst is deteriorated or not to control emission of NOx to the atmosphere.
Japanese Patent Application Laid-Open No. 2003-201888 discloses a technology of correcting a rich spike pattern taking into consideration the degree of deterioration of the catalyst.
Japanese Patent Application Laid-Open No. 2002-115536 discloses a technology concerning determination of the degree of deterioration of the catalyst based on estimation of the SOx accumulation amount.